The present invention relates to a mark position detection system for use in a charged particle beam apparatus, and more particularly to a mark position detection system capable of detecting a mark position with high accuracy on a workpiece which is set in a charged particle beam apparatus such as an electron beam lithography apparatus for fabricating a semiconductor large scale integration circuit or the like.
An electron beam lithography apparatus is used for writing fine patterns on a mask or wafer with an electron beam. For example, in the case of direct writing, it is required to write a pattern on a wafer so that the pattern is placed accurately at a predetermined position on the wafer. For this reason, as shown in FIG. 4A, a mark 2 for positioning is previously formed in the surface of a wafer as a difference in surface level, and the wafer surface including the mark 2 is scanned with an electron beam 1 to reflect electrons from the wafer surface. The reflected electrons are detected to obtain a mark signal corresponding to the intensity of reflected electrons. The mark signal thus obtained is used for determining the position of the mark (namely, the mark position), and a pattern can be written on the wafer at a predetermined position thereof by using the mark position as a reference position.
A method of detecting a mark position with high accuracy, is disclosed in, for example, JP-A-No. 59-222930. In this method, the positions of maximum and minimum levels of a mark signal are detected, and on the basis of the above positions a specific region of the mark signal is determined so that mark edge data falling within this region is used to obtain a mark position. Another conventional method of determining a mark position will be explained below, with reference to FIGS. 4A to 4F. This method is disclosed, for example, in JP-A-No. 56-15040. When the wafer surface shown in FIG. 4A and including the mark 2 is scanned with the electron beam 1 in a direction from left to right, and the reflected electrons from the wafer surface are detected by a detector, a mark signal having such a waveform as shown in FIG. 4B is obtained. When an appropriate threshold level L.sub.o is set for the mark signal, and the mark signal is processed by using the threshold level L.sub.o, a binary signal shown in FIG. 4C is obtained. At the same time as a scanning operation is started, a counter begins to count up clock pulses for the scanning operation, and the count value of the counter is read out at each edge of the binary signal. Thus, count values P.sub.1 and P.sub.m are read out as shown in FIG. 4D. The count values P.sub.1 and P.sub.m are data for indicating edge positions of the mark, and a value (P.sub.1 -P.sub.m)/2 indicates the mark position.
In the above method, however, there arise the following problems. That is, the waveform shown in FIG. 4B is not always kept constant, but varies as shown in FIG. 4E, in subsequent processes on the basis of the generation of distortion or the introduction of noise into the mark signal. In this case, a large number of false edge position data are mixed as shown in FIG. 4F, and a mark position is erroneously calculated. As a result, pattern writing accuracy is reduced. In many cases, in order to avoid such a fatal difficulty, the standard number of edge data is previously set. Further, when the number of detected edge data agrees with the standard number, the detected edge data are judged to be effective, and a mark position is determined on the basis of the detected edge data. When a portion of the bottom of a mark signal is raised as shown in FIG. 4E due to noise or others, the mark signal traverses the threshold level L.sub.o a multiplicity of times, and a large number of edge data are obtained. Accordingly, the number of edge data exceeds the standard number. That is, in the above method, the number of effective marks is limited, and thus the writability of patterns on a wafer, that is, the production yield of a desired wafer is reduced. Further, according to the above method, it is only after a mark detecting operation has been completed that judgement such as a mark position cannot be detected or is erroneously detected is made. Accordingly, a time necessary for the mark detecting operation is wasted, and thus the throughput is reduced. All of the above problems are based upon the fact that the mark signal contains noise and the waveform of the mark signal is distorted though effective information on edges of a mark is contained in the mark signal.